More than 20 people have been killed in the latest air raids on Gaza, Palestinian officials say, as Israel continues its current offensive.

The Palestinian health ministry said most died in attacks on a house and a cafe in Khan Younis in the south, bringing the overall death toll to 80.

Militants in Gaza continued firing rockets into Israel on Thursday, with sirens sounding over southern towns.

UN Secretary General Ban Ki-moon warned the situation was "on a knife-edge".

Israel launched its operation on Tuesday, after a surge in rocket-fire amid a crackdown on Hamas members in the West Bank last month, as Israel hunted for the abductors of three Israeli teenagers.

The teenagers were found murdered, and tensions were raised further with the killing of a Palestinian teenager in a suspected revenge attack days later.

'Tap on roof'

The Israeli military said that it had attacked 108 targets since midnight and that 12 rockets had been fired at Israel, seven of them intercepted by the Iron Dome defence system.

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Media captionJames Reynolds reporting from the Kfar Aza kibbutz where the community has been "shaken" by rocket attacks

Israel says its targets in Operation Protective Edge have been militant fighters and facilities including rocket launchers, weapons stores, tunnels and command centres.

The Palestinian health ministry said 17 people including five children and three women were killed in the strikes on the house and cafe in Khan Younis.

Israel has not commented on the incidents.

At the scene: BBC's Yolande Knell in Gaza

On a normal day, the streets of Gaza City are teeming with people and cars honk their horns as they sit in traffic jams. Now they are eerily quiet. Occasionally someone strides past purposefully, or a car or ambulance races by. The shops are all shuttered.

Most people here are staying at home trying to keep safe. Some will also be catching up on sleep after a noisy night when Israeli naval ships bombarded this coastal strip, making buildings shake and babies cry.

Local television stations can hardly keep up with the pace of news from inside busy hospitals and outside demolished homes. They show shocking images of dead children being pulled from the rubble on repeat.

The increasing number of civilians killed is alarming. Some people have moved in with other family members who they deem to live in safer areas. Egypt has opened its border crossing with Gaza for casualties but otherwise there is no way to leave the Palestinian territory because of the Egyptian and Israeli blockade.

At the scene: BBC's James Reynolds on Israel's Gaza border

Shortly before 0300 ( midnight GMT) in Ashkelon, a rocket siren sounded. I woke and headed to the secure room of our hotel (joined by guests in their pyjamas). There was no all-clear siren so, after a minute or two, we guessed that the threat from the rocket had passed, and headed back to our rooms.

This morning, near the border with Gaza, my colleagues and I saw a column of black smoke in a field - a fire caused by a rocket attack. Farmers drove tractors over the flames to put out the fire.

We drove on and saw Gaza itself, a few miles away, on the horizon. We saw three jet plumes of white smoke shoot up from Gaza - rockets being fired from the Palestinian territory.

Elsewhere on Thursday:

Three people died in an Israeli air strike on a car in the west of Gaza City, Palestinian media reports said. Reuters said the victims were militants from Islamic Jihad

Three people were killed in an air strike targeting a Hamas activist in the northern town of Beit Lahiya, Palestinian officials said

The Palestinian health ministry said that in addition to the dead, some 540 people had been injured overall

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Media captionBan Ki-moon: "The lives of countless innocent civilians and the peace process itself are in the balance"

Israel says militants have fired more than 365 rockets from Gaza since Tuesday and that it has attacked about 780 targets over the same time.

The armed wing of Hamas said it had fired two M75 rockets at Tel Aviv. Israel said Iron Dome had intercepted one.

It also said three rockets had hit civilian communities in the southern Negev desert and several others struck the Netivot area.

Meanwhile, an Israeli military spokesman said an attack on a house in Khan Younis on Tuesday in which eight people were killed was "a tragedy - not what we intended", adding people had returned to the building too soon following a telephone warning.

The home was said to be that of Odeh Kaware, a local Hamas commander.

Israeli sources say a second warning was given when a projectile without a warhead was fired at the building in a tactic known as a "tap on the roof", but people went back.

"They were told to leave, they returned, and the missile was already on the way. It was too late," the Jerusalem Post newspaper quoted an Israeli security source as saying.